In recent years, flat panel display devices that use flat panel display elements such as liquid crystal panels and plasma display panels are increasingly used as display elements for image display devices such as television receivers instead of conventional cathode-ray tube displays, allowing image display devices to be made thinner. In the liquid crystal display device, a liquid crystal panel used therein does not emit light, and therefore, it is necessary to separately provide a backlight device as an illumination device. Backlight devices are largely categorized into a direct-lighting type and an edge-lighting type depending on the mechanism thereof. In order to make the liquid crystal display device even thinner, it is preferable to use an edge-lighting type backlight device, and a known example thereof is disclosed in Patent Document 1 below.